1. Field of the Invention
The invention relates to an apparatus for detecting a pre-pit signal, and more particularly to an apparatus for detecting a pre-pit signal at different laser power intensities by a single slicer.
2. Description of the Related Art
In a typical DVD-R (Digital Versatile Disk Recordable), a pre-pit is additionally formed between adjacent tracks to record the address information in order to correctly detect the frequency of the wobbling tracks. FIG. 1A shows a cross-sectional view of an optical disk. As shown in FIG. 1A, the number “2” denotes a groove, the number “3” denotes a land, and the number “4” denotes a pre-pit. Since the pre-pits have been uniformly formed over the entire surface of the DVD-R when it is manufactured, the reference addresses of the DVD-R may be reproduced from the pre-pits. Shown in FIG. 1B, a push-pull signal is generated from the signals of Sa, Sb, Sc and Sd by the equation of (Sa+Sd)−(Sb+Sc), where the Sa, Sb, Sc, and Sd represent the four detecting signal of a photo detector (PD) 8. An example of this push-pull signal is illustrated in FIG. 1C. This push-pull signal contains not only Pre-pit signal but also wobble signal due to the disc track is formed by wobble-like structure. In addition, since the power intensities of the laser beams are switched between high and low power during recording onto disc for generating the information bit “pit” and “land”, respectively. Consequently, the level of this push-pull signal depends on recording power intensity, i.e., high power period produces high signal level and low power period generates low signal level. Consequently, the apparatus for detecting the pre-pit signal has to deal with the issue of push-pull signal with being high-low power dependent during recording onto disc.
U.S. Pat. No. 5,930,222, entitled “Pre-pit detecting device and information recording apparatus employing the same”, has proposed a method for detecting the pre-pit signal at a high power state and a low power state by two sets of detection circuits. FIG. 2 shows an architecture diagram of a conventional pre-pit detection apparatus. Referring to FIG. 2, the pre-pit detection apparatus 19 includes a first comparator 34 for receiving a push-pull signal and generating a first comparison signal Sc1, a second comparator 37 for receiving the push-pull signal and generating a second comparison signal Sc2, a timing generator 43 for generating a timing control signal, and a set of logic circuit for outputting the pre-pit signal Spd according to the timing control signal. The pre-pit detection apparatus 19 further employs a first low-pass filter 32 and a first adder 33 to generate a first slicing level Sla1, and a second low-pass filter 35 and a second adder 36 to generate a second slicing level Sla2. The timing generator 43 outputs a control signal Ssp to a sample and hold circuit 31 according to different power states so as to generate the first comparison signal Sc1 by the first comparator 34 at the low power state.
The method of the above-mentioned patent may be free from being influenced by different power magnitudes of the laser beams at the different power intensities. However, the drawbacks thereof are that more circuits are needed and the cost is high. For example, an additional set of detection circuit is needed.